This invention relates to dynamoelectric machines and particularly to systems for bracing the end turns of rotors of large generators that are cooled by a gas.
While the invention may be used on newly manufactured machines, it is also well suited for retrofitting into existing machines. Large numbers of turbine generators in service today have enjoyed a long service life which is being further lengthened by modernization programs that replace selected components with new and improved ones. Older rotors can be substantially refurbished by completely removing the rotor winding from the existing core and replacing it with a new winding having new copper conductors and new insulation as well as a new end winding bracing system. Normally, end windings extend axially straight out of the rotor core and then curved in substantially arcuate portions that interconnect with other axially straight portions. To minimize wear due to thermal and rotational effects, the end winding is secured by a bracing system for uniting the respective end turns together using blocks of rigid insulating material.
The overall end turn system at each end of the rotor is covered by a solid metal retaining ring. In original manufacture of many of the generators in present service the retaining rings had radial holes for coolant gas circulation through the end turn region. However, when some machines of that type have been modernized, it has been decided to have solid retaining rings without apertures for greater mechanical security. To provide gas flow through the end turn region, a blocking system is used that allows axial flow of the coolant between adjacent end turns in the axially straight portions of the end turns. Some machines were modernized with a blocking system in which an axial block disposed generally parallel to adjacent end turn straight portions is supported between two ventilation blocks at its extremities. In order that the ventilation blocks do not prevent axial gas flow they were provided with axial apertures that communicated with an end opening of the axial block which was available for coolant gas flow. It was found after some period of service that this arrangement of blocking was subject to possible deterioration and a lack of blocking capability over an undesirably short time. The ventilation blocks, as well as the axial blocks, are molded of a resin reinforced glass fiber laminate material. Such materials have good qualities of strength and electrical insulation. However, the notches in which the axial blocks fit within the ventilation blocks promoted delamination of the ventilation blocks as a result of the normal kinds of forces that the blocking system is subjected to during operation.
By way of further background, FIG. 1 shows a generalized and simplified view of a turbine generator of a prior art type to which the present invention pertains. The machine is an AC synchronous generator with a stator core 10 and stator winding 12 illustrated for general reference. A rotor core 14 is located on a shaft 16 within the stator and has conductive coils 18 within axially running slots. The coils extend out from the core 14 at both ends of the machine forming coil end turns that include axially straight portions 18a and arcuate portions 18b interconnecting straight portions in accordance with known practice. The end turns 18a and 18b at each end of the machine are held against centrifugal forces by a retaining ring 20.
A coolant gas such as hydrogen flows axially into the end turn region along the shaft 16. In early machines the retaining ring 20 had radial apertures through which the gas flowed out from the end turns. More recently machines have been equipped with solid (i.e., unapertured) retaining rings so they are less susceptible to cracking. Consequently the gas path through the end turns is axial through the end turns and the gas exits the rotor from vent scoops after making a complete axial passage through the end turns.
The regions 19 between the axially straight end turn portions 18a are to be provided with support blocks to maintain the end turns as a secure unit. Prior art forms of such blocks will now be discussed in conjunction with FIGS. 2 and 3.
FIG. 2 shows a support block structure 22 for fitting in the regions 19 of FIG. 1. In a given region 19, a structure 22 may be used one, two, or three times depending on the axial length of the region. Block structure 22 includes an axial block 24 and a ventilation block 26 at each end of the axial block. The ventilation blocks 26 fit tightly between adjacent end turn portions 18a and have axial vent passages 26a for cooling gas. Axial block 24 has end portions 24a fitting tightly in notches 26b in the ventilation blocks 26. The axial block 24 also has intermediate end portions 24b that include a gap or recess for permitting axial gas flow (arrows) through one ventilation block and over lateral surfaces of the axial block 24 to and through the other ventilation block. The vent blocks 26 secure the coils together while the axial blocks secure the vent blocks in place. Additionally, the axial blocks take up about half the space between coils (typically they are about 0.5 in. thick for about a 1 in. coil space) and thus confine the gas flow to narrower passages so the gas flows more rapidly and heat transfer is improved.
FIG. 3 shows an overhead view of a ventilation block 26 and an axial block 24 in accordance with FIG. 2. The lines show the direction in which laminations 26c run in the ventilation block 26. Tangential movement of the axial block 24, caused by relative movement between coils from differential thermal expansion and rotation loading, causes the axial block 24 to move to the position shown in dashed lines and to pry apart the ventilation block 26 as shown. The delamination starts from the corner of the keyway cut or notch 26b in which the axial block 24 fits into the ventilation block 26. The corner is a natural crack initiator and the parallel laminations of the block allow a crack to propagate quite easily.
In another form of prior art blocking, substantially the same configuration as that of FIG. 2 is used but the laminations of the ventilation block 26 run transversely rather than axially. Delamination can still occur, however, because of cracks initiated in the corners of the notches 26b.
In accordance with the present invention, the foregoing difficulties are overcome while still retaining the ability to use components of laminated material. As before, axial blocks are located between adjacent pairs of axially straight end turn portions but now they are supported differently and provide axial gas flow paths in a different manner. In contrast to the former construction, the support blocks are now located intermediate the ends of the axial block. A single axial block may extend fully between the core and the point at which the straight portion turns into an arcuate portion. At one or more locations along the length of each of the axial blocks there is a pair of support blocks. The support blocks engage the lateral sides of the axial blocks and the adjacent end turn and also engage the radial inside of the axial block. That is, each support block has a generally L-shaped cross section with a major leg disposed between one of the lateral surfaces of an axial block and one of the adjacent pairs of axially straight end turn portions. Each support block also has a minor leg disposed radially inside the axial block, preferably in a notch therein. The support block is of molded resin-impregnated glass fiber laminated material so formed that the laminations run continuously through both the major and minor legs for avoiding any tendency for delamination at the point where the major and minor legs connect.
To allow axial gas flow, the axial blocks have vent openings transversely from side to side at each location of a pair of support blocks. These vent openings have an axial extent greater than that of the support blocks so that the coolant gas can flow axially and not be blocked by the support blocks. Also, preferably, the axial blocks have chamfered portions axially fore and aft of each of the vent openings for streamlining coolant gas flow into and out of the vent openings.
The invention avoids the delamination problem found to occur in the prior axial blocking arrangements and yet still permits easy retrofitting into existing machines, as well as being useful in new manufacture.